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With higher temperatures, the water-holding capacity of the atmosphere and evaporation into the atmosphere increase, and this favours increased climate variability, with more intense precipitation and more droughts (Trenberth et al., 2003). The hydrological cycle accelerates (Huntington, 2006). While temperatures are expected to increase everywhere over land and during all seasons of the year, although by different increments, precipitation is expected to increase globally and in many river basins, but to decrease in many others. In addition, as shown in the Working Group I Fourth Assessment Report, Chapter 10, Section (Meehl et al., 2007), precipitation may increase in one season and decrease in another. These climatic changes lead to changes in all components of the global freshwater system.

Climate-related trends of some components during the last decades have already been observed (see Table 3.1). For a number of components, for example groundwater, the lack of data makes it impossible to determine whether their state has changed in the recent past due to climate change. During recent decades, non-climatic drivers (Figure 3.1) have exerted strong pressure on freshwater systems. This has resulted in water pollution, damming of rivers, wetland drainage, reduction in streamflow, and lowering of the groundwater table (mainly due to irrigation). In comparison, climate-related changes have been small, although this is likely to be different in the future as the climate change signal becomes more evident.

Current vulnerabilities to climate are strongly correlated with climate variability, in particular precipitation variability. These vulnerabilities are largest in semi-arid and arid low-income countries, where precipitation and streamflow are concentrated over a few months, and where year-to-year variations are high (Lenton, 2004). In such regions a lack of deep groundwater wells or reservoirs (i.e., storage) leads to a high level of vulnerability to climate variability, and to the climate changes that are likely to further increase climate variability in future. In addition, river basins that are stressed due to non-climatic drivers are likely to be vulnerable to climate change. However, vulnerability to climate change exists everywhere, as water infrastructure (e.g., dikes and pipelines) has been designed for stationary climatic conditions, and water resources management has only just started to take into

Chapter 3

Freshwater resources and their management

Table 3.1. Climate-related observed trends of various components of the global freshwater system. Reference is given to Chapters 1 and 15 of this volume and to the Working Group I Fourth Assessment Report (WGIAR4) Chapter 3 (Trenberth etal., 2007) and Chapter 4 (Lemke et al., 2007).

Observed climate-related trends


Increasing over land north of 30°N over the period 1901-2005.

Decreasing over land between 10°S and 30°N after the 1970s (WGI AR4, Chapter 3, Executive summary). Increasing intensity of precipitation (WGI AR4, Chapter 3, Executive summary).


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